Abrasion & Fatigue

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Abrasion & Fatigue

Figure 4A illustrates, in drawings and photographs, the wear pattern in regular lay vs. lang lay ropes. The drawings (of a single strand) show the wire direction relative to the rope axis in both types. Dimension lines in the upper drawing set off the exposed length of one wire crown in the regular lay rope. The lower drawing shows the corresponding four wire crowns involved in the lang lay rope. The line a-b shows the relation of the wire crown to the rope axis. Although there is little difference in total contact area between rope and sheave in these two rope types, the forces and wear on the individual wires are quite different (Fig. 4B).

The fact that the wires of regular lay rope are subject to higher pressure, increases the rate of wear (abrasion and peening) of both wire and mating surface of the drum or sheave. Moreover, this higher pressure is transmitted to the interior rope structure and this, in turn, decreases fatigue resistance.

Finally, the worn crown of the regular lay wire combined with its shorter exposed length, permits the wire to spring away from the rope axis (Fig. 4B). Subsequent passage on and off a sheave or drum, results in fatigue breakage.

A note of caution: lang lay rope has two important limitations. First, if either end is not fixed, it will rotate severely when under load, and secondly, it is less able to withstand crushing action on a drum or sheave, than is regular lay rope. Hence, lang lay rope should not be operated without being secured against rotation at both ends; nor should it be operated over minimum-sized sheaves or drums under extreme loads. Additionally, poor drum winding conditions are not well tolerated by lang lay ropes.

Pre-forming is a wire rope manufacturing process wherein the strands and their wires are formed-during fabrication-to the helical shape that they will ultimately assume in the finished rope or strand.

The wire arrangement in the strands is an important determining factor in the rope’s functional characteristics, i.e., its ability to meet the operating conditions to which it will be subjected. There are many basic strand patterns around which standard wire ropes are built; a number of these are illustrated in Figure 5.

Figure 5. Four Basic Strand Patterns.

Figure 6. Combination Strand Patterns

Wire ropes are identified by a nomenclature that is referenced to: 1) the number of strands in the rope, 2) the number (nominal or exact) and arrangement of wires in each strand, and 3) a descriptive word or letter indicating the type of construction, i.e., the geometric arrangement of wires.

Cross-sections of four basic strand constructions are illustrated in Figure 5; Figure 6 shows several possible combinations of these constructions.